Based on a deep XMM-Newton observation of the young open cluster NGC6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are ... [more ▼]

Based on a deep XMM-Newton observation of the young open cluster NGC6231, we derive the main X-ray properties of its early-type star population. Among the 610 X-ray sources detected in the field, 42 are associated with early-type stars. We investigate their L[SUB]X[/SUB]/L[SUB]bol[/SUB] relationship and confirm the clear dichotomy between O- and B-type stars. The cut-off line between the two behaviours occurs at L[SUB]bol[/SUB] ~ 10[SUP]38[/SUP] erg s[SUP]-1[/SUP] as previously proposed by Berghöfer et al. (1997). The distinction between single and binary stars is not clear cut, except for the colliding wind system HD152248. The X-ray detected B-stars in NGC6231 appear to be more luminous than predicted from the Berghöfer et al. relation. Though this suggests a bimodal distribution of the B-star X-ray emission, we caution however that these results might be biased by detection limits. Finally we investigate the X-ray variability of the detected sources and we find that about 40% of the X-ray emitters in the field of view present consistent signs of variability in the EPIC instruments. This fraction is much larger than previously thought. About one third of the early-type star population, either singles or binaries, further show variability. These variations do not seem to be exclusively related to binarity and could thus not be systematically interpreted in the framework of a wind-wind collision phenomenon. [less ▲]

Aims.Our goal is to characterize AGN populations by comparing their X-ray and optical classifications within the framework of the standard orientation-based unified scheme. Methods: We present a sample of ... [more ▼]

Aims.Our goal is to characterize AGN populations by comparing their X-ray and optical classifications within the framework of the standard orientation-based unified scheme. Methods: We present a sample of 99 spectroscopically identified (R <= 22 mag) X-ray selected point sources in the XMM-LSS survey which are significantly detected (>=3sigma) in the [ 2-10] keV band with fluxes between 8 × 10[SUP]-15[/SUP] and 8 × 10[SUP]-14[/SUP] erg s[SUP]-1[/SUP] cm[SUP]-2[/SUP], and which have more than 80 counts. We have compared their X-ray and optical classifications. To this end, we performed an X-ray spectral analysis for all of these 99 X-ray sources in order to assess whether they are intrinsically absorbed or not. The X-ray classification is based on the measured intrinsic column density. The optical classification is based on the measured FWHM of the permitted emission lines, the absence of broad lines being due to obscuration within the framework of the standard AGN unified scheme. Results: Introducing the fourfold point correlation coefficient r, we find a mild correlation between the X-ray and the optical classifications (r = 0.28), as up to 32 X-ray sources out of 99 have differing X-ray and optical classifications: on one hand, 10% of the type 1 sources (7/32) present broad emission lines in their optical spectra and strong absorption (N{H}[SUP]int[/SUP] >= 10[SUP]22[/SUP] cm[SUP]-2[/SUP]) in the X-rays. These objects are highly luminous AGN lying at high redshift and thus dilution effects by the host galaxy light are totally ruled out, their discrepant nature being an intrinsic property instead. Their X-ray luminosities and redshifts distributions are consistent with those of the unabsorbed X-ray sources with broad emission lines (L{2-10} 4 × 10[SUP]44[/SUP] erg s[SUP]-1[/SUP]; z 1.9). On the other hand, 25/32 are moderate luminosity (L{2-10} <= 5 × 10[SUP]43[/SUP] erg s[SUP]-1[/SUP]) AGN, which are both unabsorbed in the X-rays and only present narrow emission lines in their optical spectra. Based on their line ratios in the optical, the majority of them have an optical spectrum which is more representative of the host galaxy rather than of a reddened AGN. We finally infer that dilution of the AGN by the host galaxy seems to account for their nature. 5/25 have been defined as Seyfert 2 based on their optical spectra. In conclusion, most of these 32 discrepant cases can be accounted for by the standard AGN unified scheme, as its predictions are not met for only 12% of the 99 X-ray sources. Table 4 and Figs. [see full text]-[see full text] are only available in electronic form at http://www.aanda.org [less ▲]

Context. Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD 93162 (equivalent to WR 25), this object has been predicted to be a colliding-wind binary system ... [more ▼]

Context. Since the discovery, with the EINSTEIN satellite, of strong X-ray emission associated with HD 93162 (equivalent to WR 25), this object has been predicted to be a colliding-wind binary system. However, radial-velocity variations that would prove the suspected binary nature have yet to be found. Aims. We spectroscopically monitored this object to investigate its possible variability to address this discordance. Methods. We compiled the largest available radial-velocity data set for this star to look for variations that might be due to binary motion. We derived radial velocities from spectroscopic data acquired mainly between 1994 and 2006, and searched these radial velocities for periodicities using different numerical methods. Results. For the first time, periodic radial-velocity variations are detected. Our analysis definitively shows that the Wolf-Rayet star WR 25 is an eccentric binary system with a probable period of about 208 days. [less ▲]

in Monthly Notices of the Royal Astronomical Society (2006), 372(2), 578-590

We present the X-ray pipeline developed for the purpose of the cluster search in the XMM Large-Scale Structure (XMM-LSS) survey. It is based on a two-stage procedure via a dedicated handling of the ... [more ▼]

We present the X-ray pipeline developed for the purpose of the cluster search in the XMM Large-Scale Structure (XMM-LSS) survey. It is based on a two-stage procedure via a dedicated handling of the Poisson nature of the signal: (i) source detection on multiresolution wavelet filtered images and (ii) source analysis by means of a maximum likelihood fit to the photon images. The source detection efficiency and characterization are studied through extensive Monte Carlo simulations. This led us to define two samples of extended sources: the C1 class that is uncontaminated, and the less restrictive C2 class that allows for 50 per cent contamination. The resulting predicted selection function is presented and the comparison to the current XMM-LSS confirmed cluster sample shows very good agreement. We arrive at average predicted source densities of about seven C1 and 12 C2 per deg(2), which is higher than any available wide field X-ray survey. We finally note a substantial deviation of the predicted redshift distribution for our samples from the one obtained using the usual assumption of a flux-limited sample. [less ▲]

We present the XMM Large-Scale Structure Survey (XMM-LSS) cluster catalogue corresponding to the Canada-France-Hawaii Telescope Legacy Survey D1 area. The list contains 13 spectroscopically confirmed, X-ray selected galaxy clusters over 0.8 deg(2) to a redshift of unity and so constitutes the highest density sample of clusters to date. Cluster X-ray bolometric luminosities range from 0.03 to 5 x 10(44) erg s(-1). In this study, we describe our catalogue construction procedure: from the detection of X-ray cluster candidates to the compilation of a spectroscopically confirmed cluster sample with an explicit selection function. The procedure further provides basic X-ray products such as cluster temperature, flux and luminosity. We detected slightly more clusters with (0.5-2.0 keV) X-ray fluxes of > 2 x 10(-14) erg s(-1) cm(-2) than we expected based on expectations from deep ROSAT surveys. We also present the luminosity-temperature relation for our nine brightest objects possessing a reliable temperature determination. The slope is in good agreement with the local relation, yet compatible with a luminosity enhancement for the 0.15 < z < 0.35 objects having 1 < T < 2 keV, a population that the XMM-LSS is identifying systematically for the first time. The present study permits the compilation of cluster samples from XMM images whose selection biases are understood. This allows, in addition to studies of large-scale structure, the systematic investigation of cluster scaling law evolution, especially for low mass X-ray groups which constitute the bulk of our observed cluster population. All cluster ancillary data (images, profiles, spectra) are made available in electronic form via the XMM-LSS cluster data base. [less ▲]

in Monthly Notices of the Royal Astronomical Society (2006), 372(2), 661-678

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature ... [more ▼]

In this second paper of the series, we pursue the analysis of the 180-ks XMM-Newton campaign towards the young open cluster NGC 6231 and we focus on its rich OB star population. We present a literature-based census of the OB stars in the field of view with more than one hundred objects, among which 30 per cent can be associated with an X-ray source. All the O-type stars are detected in the X-ray domain as soft and reasonably strong emitters. In the 0.5-10.0 keV band, their X-ray luminosities scale with their bolometric luminosities as log L-X - log L-bol = -6.912 +/- 0.153. Such a scaling law holds in the soft (0.5-1.0 keV) and intermediate (1.0-2.5 keV) bands but breaks down in the hard band. While the two colliding wind binaries in our sample clearly deviate from this scheme, the remaining O-type objects show a very limited dispersion (40 or 20 per cent according to whether 'cool' dwarfs are included or not), much smaller than that obtained from previous studies. At our detection threshold and with our sample, the sole identified mechanism that produces significant modulations in the O star X-ray emission is related to wind interaction. We thus propose that the intrinsic X-ray emission of non-peculiar O-type stars can be considered as constant for a given star. In addition, the level of X-ray emission is accurately related to the star luminosity or, equivalently, to its wind properties. Among B-type stars, the detection rate is only about 25 per cent in the subtype range B0-B4 and remains mostly uniform throughout the different subpopulations while it drops significantly at later subtypes. The associated X-ray spectra are harder than those of O-type stars. Our analysis points towards the detected emission being associated with a physical (in a multiple system) PMS companion. However, we still observe a correlation between the bolometric luminosity of the B stars and the measured X-ray luminosity. The best-fitting power law in the 0.5-10.0 keV band yields log L-X = 0.22(+/- 0.06) log L-bol + 22.8(+/- 2.4). [less ▲]

We analyse observations, spanning 15 yr, dedicated to the extreme emission-line object Hen 3-209. Our photometric data indicate that the luminosity of the star undergoes marked variations with a peak-to-peak amplitude of 0.65 mag. These variations are recurrent, with a period of 16.093 +/- 0.005 d. The spectrum of Hen 3-209 is peculiar with many different lines (H I, He I, Fe II,. showing P Cygni profiles. The line profiles are apparently changing in harmony with the photometry. The spectrum also contains [O III] lines that display a saddle profile topped by three peaks, with a maximum separation of about 600 km s(-1). Hen 3-209 is most likely an evolved luminous object suffering from mass ejection events and maybe belonging to a binary system. [less ▲]

We analyze X-ray sources detected over 4.2 pseudo-contiguous sq. deg. in the 0.5-2 keV and 2-10 keV bands down to fluxes of 2× 10[SUP]-15[/SUP] and 8× 10[SUP]-15[/SUP] erg s[SUP]-1[/SUP] cm[SUP]-2[/SUP] respectively, as part of the XMM-Newton Large Scale Structure Survey. The log N-log S in both bands shows a steep slope at bright fluxes, but agrees well with other determinations below ~2 × 10[SUP]-14[/SUP] erg s[SUP]-1[/SUP] cm[SUP]-2[/SUP]. The detected sources resolve close to 30 per cent of the X-ray background in the 2-10 keV band. We study the two-point angular clustering of point sources using nearest neighbours and correlation function statistics and find a weak, positive signal for ~1130 sources in the 0.5-2 keV band, but no correlation for ~400 sources in the 2-10 keV band below scales of 100 arcsec. A sub-sample of ~200 faint sources with hard X-ray count ratios, that is likely to be dominated by obscured AGN, does show a positive signal with the data allowing for a large angular correlation length, but only at the ~2 (3)sigma level, based on re-sampling (Poisson) statistics. We discuss possible implications and emphasize the importance of wider, complete surveys in order to fully understand the large scale structure of the X-ray sky. [less ▲]

in Bulletin of the American Astronomical Society, Vol. 38 (2006, September 01)

We describe the properties of X-ray point-like sources detected over 4.2 sq. degs. of the largest contiguous survey with XMM-Newton to date (the XMM-LSS survey) to fluxes of F[SUB]2-10 keV[/SUB] 8x10[SUP ... [more ▼]

We describe the properties of X-ray point-like sources detected over 4.2 sq. degs. of the largest contiguous survey with XMM-Newton to date (the XMM-LSS survey) to fluxes of F[SUB]2-10 keV[/SUB] 8x10[SUP]-15[/SUP] erg/s/cm[SUP]2[/SUP] and F[SUB]0.5-2 keV[/SUB] 2x10[SUP]-15[/SUP] erg/s/cm[SUP]2[/SUP] respectively. For 1200 sources in the soft band, we find a two-point angular correlation function (ACF) signal similar to previous work, but no correlation for 400 sources in the hard band. A sample of 200 faint sources with hard X-ray spectra does show a 2-3 sigma positive signal with a power-law normalization theta[SUB]0[/SUB]>40 arcsec. We discuss implications, including the fact that a large correlation length for obscured AGN is inconsistent with simple AGN Unification based on orientation only. [less ▲]

in Monthly Notices of the Royal Astronomical Society (2006), 371(1), 67-80

We present the results of an optical spectroscopic campaign on the massive binary HD 152219 located near the core of the NGC 6231 cluster. Though the primary to secondary optical brightness ratio is ... [more ▼]

We present the results of an optical spectroscopic campaign on the massive binary HD 152219 located near the core of the NGC 6231 cluster. Though the primary to secondary optical brightness ratio is probably about 10, we clearly detect the secondary spectral signature and we derive the first reliable SB2 orbital solution for the system. The orbital period is close to 4.2403 d and the orbit is slightly eccentric (e = 0.08 +/- 0.01). The system is most probably formed by an O9.5 giant and a B1-2 V-III star. We derive minimal masses of 18.6 +/- 0.3 and 7.3 +/- 0.1 M-circle dot for the primary and secondary, respectively, and we constrain the stellar radius at values about 11 and 5 R-circle dot. INTEGRAL-Optical Monitoring Camera (OMC) data reveal that HD 152219 is the third O-type eclipsing binary known in NGC 6231. In the Hertzsprung-Russell (HR) diagram, the primary component lies on the blue edge of the beta Cep-type instability strip and its spectral lines display clear profile variations that are reminiscent of those expected from non-radial pulsations. Finally, we report the analysis of XMM-Newton observations of the system. The X-ray spectrum is relatively soft and is well reproduced by a two-temperature mekal model with kT(1) = 0.26 keV and kT(2) = 0.67 keV. The X-ray flux is most probably variable on a time-scale of days. The average X-ray luminosity during our campaign is log(L-X) approximate to 31.8 (erg s(-1)), but shows fluctuations of about 10 per cent around this value. [less ▲]

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC ... [more ▼]

This paper is the first of a series dedicated to the X-ray properties of the young open cluster NGC 6231. Our data set relies on an XMM-Newton campaign of a nominal duration of 180 ks and reveals that NGC 6231 is very rich in the X-ray domain too. Indeed, 610 X-ray sources are detected in the present field of view, centered on the cluster core. The limiting sensitivity of our survey is approximately 6 x 10(-15) erg cm(-2) s(-1) but clearly depends on the location in the field of view and on the source spectrum. Using different existing catalogues, over 85% of the X-ray sources could be associated with at least one optical and/or infrared counterpart within a limited cross-correlation radius of 3" at maximum. The surface density distribution of the X-ray sources presents a slight N-S elongation. Once corrected for the spatial sensitivity variation of the EPIC instruments, the radial profile of the source surface density is well described by a King profile with a central density of about 8 sources per arcmin(2) and a core radius close to 3.1 arcmin. The distribution of the X-ray sources seems closely related to the optical source distribution. The expected number of foreground and background sources should represent about 9% of the detected sources, thus strongly suggesting that most of the observed X-ray emitters are physically belonging to NGC 6231. Finally, beside a few bright but soft objects - corresponding to the early-type stars of the cluster - most of the sources are relatively faint (similar to 5 x 10(-15) erg cm(-2) s(-1)) with an energy distribution peaked around 1.0-2.0 keV. [less ▲]

Considered as the core of the Sco OB1 association, the young open cluster NGC 6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite targeted the cluster for a nominal duration of ... [more ▼]

Considered as the core of the Sco OB1 association, the young open cluster NGC 6231 harbours a rich O-type star population. In 2001, the XMM-Newton satellite targeted the cluster for a nominal duration of about 180 ks. Thanks to the detector sensitivity, the EPIC cameras provided an unprecedented X-ray view of NGC 6231, revealing about 600 point-like sources. In this contribution, we review the main results that have been obtained thanks to this unprecedented data set. Concerning the O-type stars, we present the latest developments related to the so-called 'canonical' Lx-Lbol relation. The dispersion around this relation might actually be much smaller than previously thought. In our data set, the sole mechanism that yields a significant deviation from this scheme is wind interaction. It is also the sole mechanism that induces a significant variation of the early-type star X-ray flux. In a second part of this contribution, we probe the properties of the optically faint X-ray sources. Most of them are believed to be low mass pre-main sequence stars. Their analysis provides direct insight into the star formation history of the cluster. [less ▲]

We present X-ray and optical spectroscopic observations of twelve galaxy groups and clusters identified within the XMM Large-Scale Structure (LSS) survey. Groups and clusters are selected as extended X-ray sources from a 3.5 deg2 XMM image mosaic above a flux limit 8e-15 ergs/s/cm2 in the [0.5-2] keV energy band. Deep BVRI images and multi-object spectroscopy confirm each source as a galaxy concentration located within the redshift interval 0.29<z<0.56. We combine line-of-sight velocity dispersions with the X-ray properties of each structure computed from a two-dimensional surface brightness model and a single temperature fit to the XMM spectral data. The resulting distribution of X-ray luminosity, temperature and velocity dispersion indicate that the XMM-LSS survey is detecting low-mass clusters and galaxy groups to redshifts z < 0.6. Confirmed systems display little or no evidence for X-ray luminosity evolution at a given X-ray temperature compared to lower redshift X-ray group and cluster samples. A more complete understanding of these trends will be possible with the compilation of a statistically complete sample of galaxy groups and clusters anticipated within the continuing XMM-LSS survey. [less ▲]

in Massive Stars and High-Energy Emission in OB Associations (2005, November 01)

We present preliminary results of an observational campaign aiming at the study of the binary system LSS3074. A new lightcurve clearly indicates that the system is in contact. Recent spectroscopy broadly ... [more ▼]

We present preliminary results of an observational campaign aiming at the study of the binary system LSS3074. A new lightcurve clearly indicates that the system is in contact. Recent spectroscopy broadly confirms the previous results and the rather low masses of both components (O4f[SUP]+[/SUP] and O6-7:(f):). We further analyse an XMM-Newton observation of the object that indicates a rather hard X-ray spectrum. This is partly due to the high interstellar extinction towards the object but also to the high plasma temperature (kT Ë 1.2-1.3 keV). [less ▲]

in Massive Stars and High-Energy Emission in OB Associations (2005, November 01)

We summarize the results of a multiwavelength observing campaign on the massive eclipsing binary CPD-41° 7742, another remarkable object in the young open cluster NGC 6231. Our campaign relies on high ... [more ▼]

We summarize the results of a multiwavelength observing campaign on the massive eclipsing binary CPD-41° 7742, another remarkable object in the young open cluster NGC 6231. Our campaign relies on high resolution echelle spectroscopy, narrow-band optical photometry, and XMM-Newton X-ray observations. Combined with the spectroscopic analysis, the light curve analysis provides a direct measurement of the masses and sizes of the system components. However, the most outstanding results come from the XMM observations. Our 180 ks campaign towards NGC 6231, and CPD-41° 7742, provides an unprecedented phase coverage of such a close early-type binary. The EPIC-MOS light curves almost fully cover the 2.4 day period of the system and the brightness of the object is sufficient to yield a time resolution as tight as 1 ks. The X-ray flux presents clear variations along the orbit, that we interpret as the signature of an unusual wind interaction. We indeed expect that, in this O+B system, the dominant primary wind crashes into the secondary surface, leading to a wind-photosphere interaction. As a strong support to our interpretation, we provide a geometrical model that associates an extra X-ray emission to the secondary inner surface. Though quite simple, the present model matches the main features of the X-ray light curve. [less ▲]

in Massive Stars and High-Energy Emission in OB Associations (2005, November 01)

In this contribution, we present an overview of the main results obtained by the Liège multiwavelength (in the X-ray and optical domains) campaign on the young open cluster NGC 6231. We probe the ... [more ▼]

In this contribution, we present an overview of the main results obtained by the Liège multiwavelength (in the X-ray and optical domains) campaign on the young open cluster NGC 6231. We probe the distribution of the O star properties, and especially their binary nature. In this regard, we revise the O-type binary fraction and we briefly discuss the distribution of the binary parameters. We then present the latest developments of the canonical L[SUB]X[/SUB]-L[SUB]bol[/SUB] relation. We discuss the causes of the observed deviations from this relation and of the X-ray variability among O-type stars. Probing the population of low mass pre-main sequence stars detected in the X-ray domain, we propose a scenario for the star formation history in NGC 6231. [less ▲]

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as ... [more ▼]

We discuss the first results of our ongoing optical spectroscopic monitoring campaign of the two O-type stars 9 Sgr and HD 168112. Both objects display a non-thermal radio emission and were considered as single stars. Based on a large set of high-resolution spectra, we find that 9 Sgr is clearly an eccentric SB2 binary with an orbital period of several years. On the other hand, no evidence for radial velocity variations attributable to binary motion is found in our spectra of HD 168112. [less ▲]